In modern integrated circuits, electrical contacts are typically formed by depositing a conductive material through a hole in an insulating layer of a semiconductor device, thereby forming an electrical contact with the material below. U.S. Pat. No. 4,631,248 (Pasch, Dec. 23, 1986) describes one method of forming an electrical contact in an integrated circuit. The electrical characteristics of such a contact are heavily dependent upon the size of the hole in the insulating layer.
In some applications, it is extremely desirable to know the size of the hole in the insulating layer when a contact is formed, as this gives a great deal of information about the integrated circuit manufacturing process and the quality and properties of the resultant device. Unfortunately, in modern devices, contacts are often created with dimensions of 1 micron and less, and it becomes extremely difficult to inspect and measure their size. The small size of such contact holes makes them impossible to measure optically, requiring expensive scanning electron microscope (SEM) inspection tools. Further, these measurements must be made while the device is in process, because in the normal processing of the integrated circuit, the contact hole will either be filled or etched away. If the hole has been filled, it is obscured and cannot be measured, and if it has been etched away, the original hole will have been distorted in a manner which leaves no evidence of the contact size.
As a result, only two options are available for contact measurement with the current state of the art. The first is to examine the device while it is still in process, after the hole in the insulating layer has been made, but before it has been filled with conductor or has been otherwise altered. The second is to make cross-sections of a completed device and to examine it, again with SE tools.
The problem with the first method, whereby the device is inspected before it is completed, is that the effect of further processing steps will not be evident, and any information gathered may be misleading. In addition, it is extremely difficult to determine if there is any kind of undercut in the hole in the insulator, that is, if the hole is larger at the bottom than it is at the top, and the apparent dimensions of the hole may not give a good indication of the actual contact size.
The second method, which allows for "dissection" of a completed device, whereby the completed device is physically cross-sectioned and is examined with an SEM, is far more conclusive and accurate in its measurements, but destroys the device in the process of measurement. Often, these devices are extremely valuable and scarce, and it is difficult and expensive to sacrifice any significant number of devices in order to "fine tune" the manufacturing process.
In light of the problems with direct measurement of contact features, indirect measurement techniques which give good evidence of feature sizes become extremely attractive. However, the present state of the art provides no mechanism for measuring a contact indirectly. Some techniques are available which allow for the measurement of doping profiles, channel lengths, line widths, etc., but these are usually based either on measurement of the resistance of a bar, or on resistive ratiometric measurements and are not applicable to the problems of contact size measurement.
Even though it is possible to measure the resistance of a diffused region below an insulating layer and to determine its size, this methodology give no indication of size of a contact formed through a hole in the insulating layer, and is, in fact, almost completely independent of contact size.
U.S. Pat. No. 3,388,457 (Totta, June 18, 1968) describes a method by which contacts may be formed on an integrated circuit, such that contact resistance may be measured. While contact resistance is an extremely important "bottom-line" parameter, it is dependent upon a number of processing parameter including contact size, diffusion characteristics, contamination of the via hole during processing, etc., whose individual contributions may not be determined by this method. Since the individual contributors to contact resistance may not be determined, this method is not well suited to process tuning, other than to indicate the presence or absence of a problem.
Another resistance method of contact quality determination is described in U.S. Pat. No. 3,851,245 (Baker et al., Nov. 26, 1974). In this method, holes in addition to the contact hole are provided. These are subsequently probed electrically, giving evidence that the holes in the insulating layer are either open or not, by virtue of the measured resistance. Again, this method gives evidence of a processing problem, but does not provide enough information for process tuning.
U.S. Pat. No. 3,974,443 (Thomas, Aug. 10, 1976) describes a method of measuring line width on an integrated circuit by direct resistance measurement, and U.S. Pat. No. 4,347,479 (Cullet, Aug. 31, 1982) describes a method of measuring photolithographic tolerances by integrating resistance bridges onto the wafer by which differences in line width may be measured evidentially by determining resistance ratio differences.
A different approach to resistance measurement as it applies to doping profiles in integrated circuits is given in U.S. Pat. No. 4,456,879 (Kleinknecht, June 24, 1984), whereby a D.C. current is applied to a semiconductor device, and a high-frequency modulated laser light source is used to illuminate the semiconductor at various points. Photocurrent variations resulting from moving the point of laser illumination provide evidence of doping profile in the form of a signal related to conductivity of the semiconductor.
U.S. Pat. Nos. 4,516,071 (Buehler, May 7, 1985) and 4,672,314 (Kokkas, June 9, 1987) provide additional examples of integrated test structures whereby resistance is used to measure integrated circuit device characteristics.
While all of the aforementioned resistance (or conductance) measurement methods provide useful information about integrated circuit devices, most of them make measurements which are dependent upon a large number of different process parameters, thus making it difficult, if not impossible, to determine the contributions of any single parameter. Those methods that do make a direct measure of a feature size, use the conductivity characteristics of a uniform bar to determine a line width or channel length. None of these techniques are suitable for measurement of the actual size of a contact interface area.